Unless that is the primary reason to use Filecoin. You can not prove it is porn, but if Filecoin's primary use case is child porn, then if you have Filecoin, you are likely storing someone's child porn, and you are likely running Filecoin to access your own child porn that is stored on someone else's computer.

Remember that just running a Tor node makes the authorities suspicious about that person. There are few non-suspicious reasons for running Tor unfortunately from a government's perspective, you are either an activist fleeing some government censorship (sometimes, but I think that is rare) or doing something semi-illegal (much more often.)

There is a big risk that is where this is heading.

If the file is shared it can be publicly seen.